What are the effects of isolation in the mind?

In 2000, political scientist Robert D. Putnam released the book "Bowling Alone," in which he surveyed the declining participation in organizational groups in the United States. Rather than joining bowling leagues, the title suggests that we have refuted John Donne and become individual islands. These days, we may count a high number of Facebook friends or Twitter followers, but when it comes down to it, we have no one with whom we can go bowling.

Some people crave their alone time, but more often than we may like to admit, we get lonely. According to researcher John Cacioppo at the University of Chicago, 20 percent of all people are unhappy because of social isolation at any given moment [source: Seligman]. For decades now, researchers have tracked the effects of loneliness and isolation on our physical health. One study with mice subjects found that isolation could increase cancerous tumor growth [source: University of Chicago Medical Center]. Another study found that isolation is a risk factor for disease on par with smoking and obesity [source: Goleman]. Loneliness often leads to stress, which is a risk factor for many conditions in its own right. Researchers have had subjects estimate room temperatures after recalling a time that they were snubbed or socially excluded, and the subjects reported colder temperatures than participants that were asked to remember times with friends, thus suggesting that we can actually feel social chills [source: Association for Psychological Science].

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In recent years, Cacioppo has turned his attention to the minds of the socially isolated, and his team has found that the brains of lonely people react differently than those with strong social networks. The University of Chicago researchers showed lonely and non-lonely subjects photographs of people in both pleasant settings and unpleasant settings. When viewing the pleasant pictures, non-lonely subjects showed much more activity in a section of the brain known as the ventral striatum than the lonely subjects. The ventral striatum plays an important role in learning. It's also part of the brain's reward center, and can be stimulated by rewards like food and love. The lonely subjects displayed far less activity in this region while viewing pleasant pictures, and they also had less brain activity when shown the unpleasant pictures. When non-lonely subjects viewed the unpleasant pictures, they demonstrated activity in the temporoparietal junction, an area of the brain associated with empathy; the non-lonely subjects had a lesser response [source: University of Chicago].

What's still unclear is whether loneliness rewires the brain to function differently, or whether the brain predisposes certain people to feel isolated. Is loneliness a result of not being able to see the rewards right in front of you? While researchers suss out such questions, the study and others like it do have some immediate implications in the field of mental health. In 2008, University of Michigan researchers evaluated the relationship between social isolation and mental functioning. They conducted a study that measured memory and intellectual performance in groups that had just spent 10 minutes socializing as well as groups that spent 10 minutes reading and completing crossword puzzles. The groups that spent their time interacting with others performed just as well as the groups that were essentially warming up their brains [source: University of Michigan]. Perhaps such results can be explained by Cacioppo's research; the ventral striatum is, after all, associated with learning. So the next time you're reaching for a crossword puzzle to keep your brain sharp, it might do just as much good to reach out to a friend instead.